1. Field of the Invention
The invention relates to a method and a receiver circuit for reducing RFI interference in a DMT data transmission.
2. Description of the Related Art
High-bit-rate data transmission on a subscriber line is increasingly becoming more important in modern telecommunications, particularly because it is anticipated to yield a larger useable bandwidth of the data to be transmitted combined with bidirectional data communication.
One technique which has recently been gaining more and more in importance is so-called multicarrier data transmission, also known as “Multicarrier” transmission, as “Discrete Multi-tone” transmission (DMT) or as “Orthogonal Frequency Division Multiplexing” transmission (OFDM). DMT transmission is suitable particularly for data transmission via linearly distorted channels. Compared with single-carrier data transmission, advantages are also afforded with regard to flexibility in adapting the data rate or the transmission spectrum to the transmission channel or to the interference environment. DMT transmission is used for example in conductor-based systems, but also in the field of radio, for broadcast systems and for access to data networks. Examples of applications of DMT transmission are digital radio broadcasting (DAB=Digital Audio Broadcast) and digital television (DVB=Digital Video Broadcast).
One representative of DMT transmission is the ADSL technique, for example, where ADSL stands for “Asymmetric Digital Subscriber Line”, that is to say asymmetric digital subscriber access connection via a normal telephone line. ADSL denotes a technique that permits the transmission of a high-bit-rate bit stream from a central station to a subscriber and of a low-bit-rate bit stream passing from the subscriber to a central station. This technique involves subdividing the telecommunication line into at least one channel for conventional telephone services (that is to say voice transmission) and at least one further channel for data transmission. In addition to the ADSL technique there are also further representatives of the so-called xDSL technique, thus for example broadband subscriber access connection (VDSL=Very High Speed Digital Subscriber Line).
DMT transmission systems use a multiplicity of carrier frequencies, the data stream to be transmitted being decomposed into many parallel partial streams that are transmitted by frequency division multiplexing. These partial streams are also referred to as individual carriers. For the modulation, the transmission signal is composed of many sinusoidal signals, each individual sinusoidal signal being modulated both in terms of amplitude and in terms of phase. A multiplicity of quadrature-amplitude-modulated signals are thus obtained. For DMT transmission, an IFFT transformation is used in the transmitter and an FFT transformation is used in the receiver. Efficient and fast signal processing algorithms exist both for the IFFT and for the FFT.
One problem that is primarily associated with very broadband DMT transmission results from the fact that the useable frequency range extends to above 12 MHz or more. This wide frequency range, however also contains frequency bands for other services such as, for example, the frequency bands for medium-wave and short-wave radio broadcasting or the frequency ranges for amateur radio. Depending on the spatial position, the subscriber terminals of DMT transmission may be disturbed by these services that use the same frequency range of DMT transmission. The term employed here is so-called RFI interference, where RFI stands for “Radio Frequency Interference”. This RFI interference represents narrowband interferences relative to the very broadband frequency range utilized for DMT transmission, since these interfering RFI frequency ranges typically have a width of just a few kilohertz.
If such a narrowband interference signal is superposed on the DMT reception signal, the demodulated DMT reception signal is then adversely affected thereby. Not only the values (carrier frequencies) in the immediate vicinity of the centroid frequency of this interference are disturbed in this case. Carrier frequencies (or useful channels) of the DMT reception signal that are much further away from this interference frequency are also disturbed.
This shall be explained on the basis of an example in the case of a VDSL data transmission and an assumed purely sinusoidal interference having the frequency f0=1211.1 kHz. The DMT receiver demodulates the received DMT reception signal with the aid of an FFT transformation. In accordance with the VDSL standard, the frequency spacing of the individual synchronous carrier frequencies is precisely Δf=4.3125 kHz. The interference frequency f0 thus corresponds to none of the carrier frequencies used in the DMT transmission, that is to say that the interference frequency is asynchronous with respect to the carrier frequencies of the DMT transmission.
FIG. 1 shows the normalized power density Pst of an interference signal having the frequency f0=1211.1 kHz after the FFT demodulation in the vicinity of the normalized interference frequency f0/Δf≈281 as a function of the carrier frequency f. The carrier frequency f is normalized here to the frequency spacing Δf. FIG. 1 illustrates the resulting interference spectrum—normalized to the maximum value of the interference frequency f0/Δf—in dB. It is evident that, without further measures, the interference spectrum has decayed by approximately 50 dB below and above the interference frequency f0/Δf only after approximately 50 carrier frequencies (f/Δf). This means that useful channels of the DMT reception signal which are relatively far away from the centroid frequency f0/Δf of the interference signal relative to the bandwidth of the interference signal are still disturbed by precisely this interference. This is due to the fact that the narrowband interference signal is typically not present orthogonally or synchronously with respect to the carrier frequencies utilized for the transmission and is thus not completely eliminated either.
The RFI interference, as already mentioned above, comprises relatively narrowband interferences attributable for example to radio broadcasting waves or to amateur radio waves. In medium-wave or short-wave radio broadcasting, the transmitted signals are modulated with the aid of double-sideband amplitude modulation with carriers and emitted. Consequently, a permanent interference having a constant centre frequency is to be reckoned with here (interference of a first type). By contrast, amateur radio involves the use of single-sideband amplitude modulation with a suppressed carrier. Therefore, no permanent interference occurs here (interference of a second type). Rather, the interference here is dependent on the amateur radio transmitter, that is to say the speaker.
The article “A Narrow-Band Interference Canceller for OFDM-based Systems” from Rickard Nilsson, Frank Sjöberg and James P. Leblanc in Proceedings of 4th European Personal Mobile Communications Conference (EPMCC 2001), Vienna, 17-22 February 2001, describes a method for solving this problem. This method deals with the occurrence both of interference of the first type and of interference of the second type. In the method described therein, it is assumed that the carrier frequencies in the immediate vicinity of the interference frequency, in the so-called observation interval, are not utilized for the data transmission. For this purpose, after the FFT demodulation in the receiver, firstly the precise spectral position of the interference signal is determined. By means of suitable processing of the complex frequency values in the observation interval in which the interference frequency is also situated and which contains no portions of the useful signal, it is possible to generate a compensation signal in the form of a complex signal vector. This complex signal vector is subsequently subtracted from the demodulated signal vector of the data transmission.
In the case of this method, however, generating the compensation signal presupposes exact a priori information about the received interference signal, for example in the form of a spectral power density of the interference signal that is as exact as possible and is determined or known at the outset. Furthermore, knowledge that is as accurate as possible about the type and the profile, that is to say the form, of the interference signal is required, that is to say whether the interference is sinusoidal, for example. This method then attempts to simulate the profile of the interference outside the observation interval as accurately as possible, which is very complicated if only for this reason. This means, however, that the applicability, the performance and thus the usefulness of this known method are significantly restricted. Moreover, the realization and implementation of this method requires an exorbitantly high outlay on hardware and software for determining the a priori information and also for calculating the corresponding compensation signals.